1. Field of the Invention
This invention relates to articles having ceramic-paint coatings and to methods of making such articles.
2. Description of the Prior Art
There are several known kinds of water-base or oil-base ceramic paint, each of proprietary composition. For many purposes, such paint is unsatisfactory when applied as a single coat. Accordingly, it has occurred to those skilled in the art that best results might be obtained by using more than one coat. Efforts have been made to apply a second coat of paint, but it has developed that there is a severe problem in getting the second coat of ceramic paint to adhere to the first. Many approaches were tried, including roughening of the first coat, slower baking, using heavier coats, using lighter coats, and treatments of the surface of the first coat with various media, all these approaches proving to no avail. Thus, even when the ceramic paint has been applied with the greatest of care, it has frequently been difficult or impossible to avoid the occurrence of one or more small pinholes in the coating that cause it to fail. The likelihood of such pinholes is increased if the ceramic paint is cured rapidly, so that water or oil becomes trapped beneath an early-hardening exterior layer of the ceramic paint; at the same time, the use of slow curing at relatively low temperatures for prolonged times is relatively expensive, and at that, it does not provide a complete answer to the problem.
Yet another drawback of most of the known ceramic paints is that after having been applied and cured, they must be cooled rather slowly to room temperature, as they will crack if simply removed from the furnace in which they are cured and permitted to air-cool. This adds undesirably to the time and expense involved in protecting an article with the use of such ceramic paints, and it would accordingly be advantageous to provide a ceramic-paint-base coating that would withstand the thermal shock of an air-cool without exhibiting such cracking.